1. Field of the Invention
The invention relates to a composite material made of a high-strength aluminium alloy, and more particularly to a composite material used for the production of brazed heat exchangers used in vehicles.
2. Related Technology
Development of high-strength aluminium alloys is forced by the demand of the vehicle industry to manufacture lighter and thinner-walled materials.
While aluminium alloys or components made of aluminium alloys naturally have a good corrosion resistance, their mechanical strength is low. Adding alloying constituents such as magnesium (Mg) enhances, on the one hand, the mechanical strength of the alloy, but the corrosion resistance after brazing is problematic. Particularly aluminium alloys with magnesium contents of more than 0.3% and potassium aluminiumfluoride, e.g. NOCOLOK, is used as brazing material, considerable problems arise during brazing. Aluminium alloys with such a high magnesium content have a low corrosion protective potential due to the diffusion of some alloying constituents which have an electrochemical potential different from the matrix of the aluminium alloy.
When aluminium alloys are used for brazed structures such as heat exchangers, the brazing material has a cathodic effect on the aluminium alloy used as base material so that electrochemical corrosion is facilitated. Moreover, there is a risk for brazing materials containing silicon that the silicon intergranularly diffuses into the aluminium alloy, which results in grain boundary corrosion.
On principle, the corrosion risk of components made of an aluminium alloy depends on the alloying constituents, the production process, the site of use and the environmental conditions governing there.
From the state-of-the-art, different methods are used to increase the corrosion stability of aluminium alloys.
The addition of zinc can modify the brazing material so that it has an anodic effect. In this case the corrosion current flows from the more active coating material to the aluminium alloy. In time, the modified brazing material, however, acts as a sacrificial anode so that the aluminium alloy may corrode. On the other hand, a cathodically acting aluminium alloy is obtained when another element is added to the aluminium alloy as an alloying constituent. Moreover, the aluminium alloy used as base material can be improved in this way, in order to prevent the intergranular, or intercrystalline, respectively, diffusion of silicon.
DE 28 18 564 A1 discloses a method for corrosion protection of aluminium tubes for heat exchangers and correspondingly produced heat exchangers. A heat exchanger which is at least partly made of aluminium or an aluminium alloy is coated with a protective alloy. The protective alloy is, according to the patent filing data, an aluminium alloy containing up to 12% silicon.
US 2002/0142185 A1 discloses a 4-layer aluminium alloy for use in radiators. Two intermediate layers are provided which are used to enhance the brazing behaviour of the core layer. From these intermediate layers silicon can diffuse into the core layer in order to improve the strength of the core layer. It is a disadvantage of this invention that the core layer has low corrosion stability.
In DE 31 27 980 C2 a composite material for tubes of brazed heat exchangers and the use of this composite material are described. The composite material is a plated aluminium core alloy with the plating material consisting of pure aluminium, an aluminium-manganese alloy or an aluminium-silicon alloy with a copper content of 0.2% maximum. It is characteristic of this invention that the core alloy consists of 0.2 to 2% copper and aluminium and impurities as balance. In addition, the core alloy can contain 0.01 to 0.5% zirconium, 0.05 to 0.5% manganese and 0.05 to 0.5% chromium.
It is the object of this invention to provide a composite material, particularly for tubes of heat exchangers, which has a low mass, long life, high strength and high corrosion resistance.